Re: Directional Derivatives

If a function is differential at a point, then it must be continuous there. The "contrapositive" is that if a function is not differentiable at a point, then it is not differentiable there. Clearly, the limit, as you approach (0, 0) along the lines x= 0 or y= 0, is 0. Look at the limit as you approach (0, 0) along the line

Re: Directional Derivatives

Originally Posted by HallsofIvy

If a function is differential at a point, then it must be continuous there. The "contrapositive" is that if a function is not differentiable at a point, then it is not differentiable there. Clearly, the limit, as you approach (0, 0) along the lines x= 0 or y= 0, is 0. Look at the limit as you approach (0, 0) along the line

Ok so here is the solution I have come up with:

Problem:
Prove that the function

has directional derivative at the point in any direction. Is the function differentiable at ? Justify your answer.

Solution: is not continuous at therefore it is not differentiable at . We will show that the directional derivative at the point exists in any direction. Let and .Then we have:

.

Therefore and when . So the directional derivative exists at in any direction.

Re: Directional Derivatives

You did not show that the function is not continuous at , but you did show that the derivative does not exist. You mixed up the subscripts for your 's. The result when should be . Your function was , not . Also, I don't understand the notation: . To me, that reads the derivative with respect to zero at the point . Since zero is a constant, not a variable, I don't think that derivative is well-defined. Once you show that the limit changes as you approach from different directions, you have sufficiently showed that the derivative does not exist at .

Re: Directional Derivatives

Originally Posted by SlipEternal

You did not show that the function is not continuous at , but you did show that the derivative does not exist. You mixed up the subscripts for your 's. The result when should be . Your function was , not . Also, I don't understand the notation: . To me, that reads the derivative with respect to zero at the point . Since zero is a constant, not a variable, I don't think that derivative is well-defined. Once you show that the limit changes as you approach from different directions, you have sufficiently showed that the derivative does not exist at .

Sorry it should read is supposed to refer to the directional derivative when . I am trying to show that the directional derivative exists at in any direction using the definition that for any unit vector , . This limit, if it exists, is called the directional derivative of at p in the direction .

is continuous when the denominator is nonzero. We can find when the denominator is zero by setting . This can only happen when , so is continuous away from the origin, i.e., it has a discontinuity at .

Re: Directional Derivatives

Originally Posted by SlipEternal

So, you are calculating the directional derivative at the point , not at .

Ok I think I am calculating correctly but with bad notation, let me redo this a bit:

is continuous when the denominator is nonzero. We can find when the denominator is zero by setting . This can only happen when , so is continuous away from the origin, i.e., it has a discontinuity at . Since is not continuous at therefore it is not differentiable at .

Now we will show that the directional derivative at the point exists in any direction. Let and . Using the definition:

For any unit vector , . This limit, if it exists, is called the directional derivative of at p in the direction , we can write:

Re: Directional Derivatives

does not have a denominator when . It is defined to be zero at . So, while it may be true that the function is not continuous at , you have not proven it. It is difficult to show that a multivariable function is discontinuous at a point, but not impossible. If you prove that the function is not continuous at , then you are done. You don't need to check directional derivatives at all. But, if you are not going to prove that, then you should remove the statement that it is discontinuous at that point.

You show that while the directional derivatives exist, they are not equal. So, the derivative does not exist.

Re: Directional Derivatives

If you want to show that is not continuous at , you can do that. To show that the function is continuous, you would want to show that . So, as HallsofIvy suggested consider . As , , so if this limit does not yield 0, then is not continuous at . If it is not continuous, then the derivative does not exist. Note: what you are doing above is the same as single variable calculations for derivative. You can show and both exist, but if they are not equal, then the derivative does not exist. Those would be the single variable directional derivatives. Anyway, to show that is not continuous at , we have: